With the ever-increasing oil price, fuel economic efficiency and fuel-saving potentials of an engine are becoming more and more important. Recently, most fuel-saving researches are focused upon developing variable valve actuation mechanism since it is the foundation of various fuel-saving techniques, such as cylinder deactivation, engine down-sizing, and so on.
Fuel-saving can be achieved by changing valves' lift, which is realized by methods listed as following:                (1) Designing intake valves to synchronously enable a high or a low lift selected with respect to engine speeds: The valve lifts of two intake valves of an engine are optimized for matching the same with the engine speed, by which high valve lift is adopted for enhancing intake efficiency and thus helping to develop high-power output with less fuel consumption when the engine is operating at high speeds, and low valve lift is adopted when the engine is operating at low/median speeds for reducing fuel consumption since the intake flow speed is increased, the driving torque of camshaft is reduced and the combustion of the engine at idle is stabilized. The aforesaid design is commonly being adopted by Honda and used in its products, such as CIVIC and ACCORD. In addition, The Valvetronic system of BMW is the first variable valve timing system to offer continuously variable intake valve lift for optimizing the performance of engines.        (2) Designing one of two intake valves to enable a high lift while another enabling a low lift: Such design basically allows only one intake valve to be opened for intaking air when an engine is operating at a low/median speed, by which an intense swirl can be created inside its cylinder so as to improve combustion efficiency and thus improve fuel consumption. It is noted that when the engine is operating at high speeds, both of the two intake valves are enabled to perform at a high valve lift. The CB400F of Honda is the representative of such design. However, in order to avoid fuel from depositing at the closed intake valve when the engine is operating at a low/median speed and thus cause troubles, such as incorrect air/fuel ratio and carbon deposition, one intake valve is enabled with a high lift while another is enabled with a low lift.        (3) Deactivating partial valves from intaking: For large-volume engine or hybrid engine, it is preferred to reduce pump loss during cylinder deactivation that can be achieved by designing valves of a portion of a cylinder to be closed when the engine is operating at low speed. The Insight of Honda is the representative of such design.        
Currently, there are various researches relates to valve lift control. One such research is disclosed in U.S. Pat. No. 4,523,550, which uses a valve actuation mechanism with adjustable valve disabling device for valve lift control, and is the design capable of enabling one of two intake valves with a high lift while another with a low lift, or enabling only one valve is opened while another is closed. Another such research is disclosed in U.S. Pat. No. 4,727,831, which uses the combinations of three cams and three rocker arms for controlling two valves capable of selectively operating in two operation modes, that is, enabling both valves with a high lift synchronously or enabling one of two intake valves with a high lift while another with a low lift. Further another such research is disclosed in U.S. Pat. No. 4,887,563, which uses the combinations of three cams and three rocker arms for controlling two valves capable of selectively operating in three operation modes, that is, enabling both valves with a high lift synchronously, or enabling one of two intake valves with a high lift while another with a low lift, or enabling one of two intake valves with a median lift while another with a low lift.
Although methods and apparatuses disclosed in the aforesaid patents are all capable of providing multiple operation modes of valve lift, they are all short for providing valve lift control capable of meeting every operation requirement of an engine as it is operating at a high speed for high-power output, or as it is operating at a median speed and requiring an vertex inside its cylinder for improving combustion efficiency, or as it is subject to a cylinder deactivation condition, or as it is stalled. Therefore, it is in need of a valve actuation mechanism that is freed from the shortcomings of prior arts.